This invention relates to a control circuit, and more particularly, to a control circuit which operates at a high speed in substantially instantaneous response to ON-OFF operations of an AC power source to produce a control signal which can be used to energize or de-energize a device in an electronic apparatus.
In some electronic apparatus there often is a problem associated with the turning on or off of input power. This problem is caused by the current peaks which accompany the initial supply of power or the abrupt termination thereof. For example, in audio apparatus having a sound transducer, such as a loudspeaker, driven by an amplifier, a so-called "popping" noise often is produced a short time after AC power initially is supplied to the apparatus, or after such AC power is cut off. This popping noise is produced in response to the discharge of capacitors included in the circuits of the audio apparatus. When the popping noise signal is supplied to the loudspeaker, a distracting and audibly uncomfortable sound is generated. Under certain circumstances, the loudspeaker can be damaged by the popping noise and its associated current peaks.
In order to avoid supplying the popping noise and current peaks to the loudspeaker, a special switch ganged with the usual power switch is connected in a circuit coupled with the loudspeaker. This current coupled to the loudspeaker is opened by the special switch, simultaneously with the turning off of the power switch. Although this circuit arrangement is, in most instances, satisfactory, its preventive function is effectively by-passed if the power supply cord for the apparatus is separated from the power source, e.g. pulled from its plug-socket. In that instance, which is not infrequent, the popping noise cannot be avoided.
In another arrangement, the ripple voltage of the DC power source of the apparatus is detected to drive a control signal. This control signal is used as a muting signal to open and close a gate circuit coupled to the loudspeaker. However, since the DC power supply circuit usually includes a capacitor of high capacitance, and thus a large time constant, the ripple voltage will be detected following a relatively long time delay after the power switch is turned on or turned off. Consequently, the gate circuit will not be closed until the ripple voltage is detected, and the opened gate circuit will allow the popping noise to be produced when the power switch is operated.